GiST stands for Generalized
Search Tree. It is a balanced, tree-structured access method,
that acts as a base template in which to implement arbitrary
indexing schemes. B-trees, R-trees and many other indexing
schemes can be implemented in GiST.

One advantage of GiST is
that it allows the development of custom data types with the
appropriate access methods, by an expert in the domain of the
data type, rather than a database expert.

Some of the information here is derived from the University of
California at Berkeley's GiST Indexing Project web site and
Marcel Kornacker's thesis, Access Methods for Next-Generation Database
Systems. The GiST
implementation in PostgreSQL is
primarily maintained by Teodor Sigaev and Oleg Bartunov, and
there is more information on their web
site.